Polyolefins, such as polyethylene (PE), are synthesized by polymerizing monomers, such as ethylene (CH2═CH2). Because they are cheap, safe and stable to most environments and easy to be processed, polyolefins are useful in many applications. Polyethylene can be classified into several types, such as, but not limited to, LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene) as well as High Molecular Weight (HMW), Medium Molecular Weight (MMW) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.
Olefin (such as ethylene) polymerizations are frequently carried out in a loop reactor using monomer (such as ethylene), diluent and catalyst, optionally an activating agent, optionally one or more co-monomer(s), and optionally hydrogen.
Polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles suspended in diluent. The slurry is circulated continuously in the reactor with a pump to maintain efficient suspension of the solid polymer particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs. Settling in the legs is used to increase the solid concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer may be extruded and pelletized.
Optimal behavior of the settling legs is reached when the quantity of recovered solid polymer is maximized with respect to the amount of fluid effluent that must be recycled, so that the recycling cost may be minimized for a given production rate. The use of settling legs, however, results only in a minor increase in the concentration of solid polymer withdrawn form the polymerization loop reactor.
Various alternative product removal techniques are known. For example, by means of a continuous product take off, more in particular by means of an elongated hollow appendage provided on the reactor, said hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.
WO 2004/024781 describes a slurry polymerization process which removes a portion of the fluid slurry without using a settling leg or a continuous take-off. The process uses a reactor take-off valve that is periodically fully closed and fully opened such that the withdrawn slurry is removed from the reactor in a discontinuous manner.
WO 01/05842 describes an apparatus for removing concentrated slurry from a flowing stream of slurry in a conduit characterized by a channel in an outlet area of the conduit, the outlet being adapted to continuously remove slurry.
The above-described known processes and apparatuses have the disadvantage that the product slurry withdrawn from the reactor still contains a large amount of diluent and other reactants, such as monomer, which implies the necessity to subsequently separate them from the solid polymer particles and to reprocess them for the purpose of reusing them in the reactor.
There remains a need in the art for an improved polyolefin production process. It is an object of the present invention to provide an improved polyethylene preparation process, wherein the product slurry withdrawn from the reactor contains an increased concentration of solid polymer particles and a decreased concentration of diluent and other reactants, such as monomer. It is further an object of the present invention to provide a loop reactor having improved operating conditions.